Since the Renaissance Period artists have used canvas stretched taut over a wood frame as their preferred surface for painting. Even then it was known that stretched canvas changes dimensionally with temperature and humidity change and must be re-stretched periodically to maintain proper tension on the painted canvas surface. If not maintained, slack and sag in the canvas over time will cause cracking in the paint surface and delamination of the paint from the canvas. This delamination is very detrimental to the preservation of these works of fine art. Restretching a painting on canvas initially required removing the numerous tacks which secured the canvas to the wood frame and then re-attaching the canvas again to the same or slightly smaller frame to achieve the proper surface tension. This is an elaborate process which often caused much damage to the edges of the canvas. Various methods subsequently have been developed to re-tension an art canvas without removing it from its stretcher frame. A brief description of this prior art follows below.
U.S. Pat. No. 1,847,925 discloses a frame with double interlocking tendon/mortise joints, wherein two ‘keys’ (small wedges) at each corner are tapped deeper into the mortice joints to separate the frame corner. These keys can loosen and fall out unless tacks are used to secure the wedged corner once the proper canvas tension is achieved. These tacks must then be removed to re-tension the canvas at a later date, again using tools and manipulations that endanger the artwork.
U.S. Pat. No. 3,485,165 discloses a silk screen frame with a traveling slide at each corner that stretches the silk using a bolt to pull the slide.
U.S. Pat. No. 3,529,653 discloses an expandable picture frame that uses a metal tensioning brace mounted diagonally across a corner. By adjusting a bolt that travels in a slotted opening, future adjustments can be made by loosening the bolt, and re-securing the brace, using a wrench.
U.S. Pat. No. 3,882,616 discloses a dual wedge frame tensioner but the wedges interlock. A hammer must be used to adjust the wedges.
U.S. Pat. No. 3,949,802 discloses a pair of metal brackets fastened at each corner of a standard mitered picture frame. Each bracket has an adjusting screw to push the frame corners apart. A screwdriver can be used for future re-tightening.
U.S. Pat. No. 4,144,660 discloses a picture frame corner that consists of a square block. Each frame arm projecting from the block can be tensioned by hand tightening a traveling nut mounted on a bolt imbedded into the corner block. No tools are needed to re-adjust the tension in the future.
U.S. Pat. No. 4,301,853 discloses a pair of metal corner brackets attached to a wooden frame. A threaded rod on each bracket is tightened by a wrench to adjust the tension in the corner. A spring biases the tension, thus spring fatigue could loosen the canvas.
U.S. Pat. No. 4,549,596 discloses a combination spring and screw adjuster to tension an insert in a hollow of a frame corner, thus spring fatigue could loosen the canvas.
U.S. Pat. No. 4,635,700 discloses a multi-spring two dimensional tensioning picture frame. Each spring can be adjusted for tension.
U.S. Pat. No. 4,829,685 discloses a rotatable cam at each corner of a tensioning frame. A screwdriver can adjust each corner's tension.
U.S. Pat. No. 4,947,922 discloses a coil spring imbedded in each corner to tension the canvas.
U.S. Pat. No. 5,052,462 discloses a frusto conical cone which spreads two disks apart with a screwdriver.
U.S. Pat. No. 5,115,584 discloses a frame stretcher which uses a series of insertable rods to stretch the frame.
U.S. Pat. No. 5,900,276 discloses the use of a water based adhesive to re-tighten a canvas.
U.S. Pat. No. 6,347,466 discloses a thermoplastic heat shrinking canvas method.
What is needed in the art is a simple stretcher frame without springs that can open (expand) the corners of the frame merely by twisting a wing nut or knob. The present invention integrates a reverse action corner embedment at each corner joint to accomplish these objectives.